Among 40–79 year old African Americans without baseline CHD and stroke, those in the highest HbA1c PRS tertile had higher percentage-point in 10-year predicted ASCVD risk and higher odds of ≥ 10% 10-year predicted ASCVD risk compared with those in the lowest HbA1c PRS tertile. In addition, individuals in the highest SBP PRS tertile had higher odds of ≥ 10% 10-year predicted ASCVD risk compared with those in the lowest SBP PRS tertile. Correspondingly, individuals in the highest DBP, LDL cholesterol, triglycerides and CRP PRS tertiles had higher but not statistically significant 10-year predicted ASCVD risks compared with those in lowest PRS tertiles. The associations of HbA1c and SBP PRSs with 10-year predicted ASCVD risk were independent of participants’ age, sex, study visit date and genetic ancestry, suggesting utility of PRSs for capturing long-term exposure to a heritable CVD risk.
Our study evaluated PRSs estimated in a large African-ancestry GWAS within an independent but limited sample size of African-Americans. The association between higher SBP PRS with incident CVD was recently reported and validated in a large pooled data that included participants from multiple ancestries.[18] Several prior studies have also reported associations of blood pressure PRSs with CVD risk,[41–44] but even in pan-ancestry GWASs, European-ancestry groups remain over-represented. In the present study, we also observed that HbA1c and SBP PRSs derived from European-ancestry UKBB data were associated with 10-year predicted ASCVD risk in the JHS, owing to the large sample of the UKBB GWAS. Though pan-ancestry data sources have provided statistical power to evaluate PRSs while providing the potential to address transferability of polygenic risk prediction across different ancestries,[45] several challenges have also been underscored.
First, the overestimation of CVD risk in non-European ancestries were noted when applying prediction models derived from pooled cohorts over-represented by a specific-ancestry.[7, 46] Due to allele frequency differences,[9] hypertension PRS distributions, for example, differed by proportion of specific genetic ancestry.[13] While African genetic ancestry of African Americans ranges from 30–100%, genetic heterogeneity by admixture may influence the sums of alleles, which are used to construct PRSs. Second, when PRS distributions differ across race/ethnicities, PRSs based on pan-ancestry GWAS data pose a challenge on predicting individuals “at risk” for cardiometabolic factors and CVD.[13, 47] For example, individuals may be misclassified as “at risk” when admixed individuals are not accurately represented by any specific PRS distribution. Alternative strategies to overcome the challenge include CVD risk prediction tailored towards specific groups, such as African American women and men,[1, 6] who are disproportionately affected by CVD.[9–13] Current guidelines also suggest population-specific genetic investigations to reduce disparities and guide disease prevention efforts in the context of multi-factorial complex diseases like CVD.[1]
Third, studies with flexible risk prediction models that account for non-genetic risk factors and their interactions have been limited but are critical to reducing disparities and guiding disease prevention efforts.[9–13] For example, a recent study highlighted candidate SNPs that modified the association between perceived discrimination and elevated SBP in the JHS.[48] In the present study, HbA1c and SBP PRSs were associated with 10-year predicted ASCVD risk after additionally accounting for smoking and insurance statuses (data not shown), however, the associations were not modified by smoking or insurance statuses. Socio-economic stressors that disproportionately affect population groups are relevant and larger studies that evaluate PRS-by-environment interactions,[13, 48] while taking careful considerations with respect to how the sample populations’ genetic ancestry compares to the that of the training data are needed.[12]
Furthermore, the role that polygenic risks for elevated lipid and inflammatory factors, as cardiometabolic markers of plaque stability, play on CVD risk among African-ancestry individuals is unknown. Evaluating lipid and inflammatory marker PRSs in CVD risk may address risk misclassification due to genetic influences on the performances of their assays and allow investigation into pleiotropy between the traits. For example, genetic factors of hemoglobin influenced the performance of some HbA1c assays, potentially leading to misclassification of individuals who have achieved glucose control and may be at lower risk for type-2 diabetes.[10, 49, 50] Therefore, the lack of knowledge regarding genetic variants affecting HbA1c measurement independently of blood glucose concentration may exacerbate health disparities due to misdiagnosis and treatment inaccuracy.[51]
The associations of HbA1c genetic variants with type-2 diabetes are known,[51, 52] including in a recent study that demonstrated transferability of the findings in individuals that share African-ancestry.[10] However, a 1%-unit increase in HbA1c was also associated with a 20–50% increased CVD risk in individuals without type-2 diabetes.[53] The prevalence of type-2 diabetes in the JHS participants included in this study is 24%, which is higher than the national type-2 diabetes prevalence in African Americans, but HbA1c levels may also contribute to CVD risk in diabetic individuals. HbA1c increases dyslipidemia, hypertension, CRP, oxidative stress and blood viscosity leading to CVD.[54] Among European-ancestry individuals, a mendelian randomization study has shown that known HbA1c SNPs are associated with CVD risk.[55]
In European-ancestry individuals, a Mendelian randomization study suggested a causal association between cholesterol SNPs and type-2 diabetes.[56] Triglyceride is not directly atherogenic but represents an important biomarker of CVD risk because of its association with atherogenic remnant particles.[57] Other inflammatory markers like CRP can induce inflammatory changes in endothelial and smooth muscle cells and are related to CVD risk.[58] Although we were unable to assess the genetic overlap between HbA1c, cholesterol, triglycerides in our study, methods appropriate in leveraging pleiotropy in larger studies have been developed.[59, 60] These methods may enhance both discovery and genetic associations while creating potentially more powerful PRS for each of the traits.[13]
Lastly, our study has several other limitations and strengths. The sample size for African-origin UKBB participants ranged from 5,290–6,551, representing a small sample size for detecting genome-wide significant common variants. However, the UKBB is the largest publicly available GWAS summary data with representative samples across various ancestries. Furthermore, our study excluded < 40 years old individuals because 10-year predicted ASCVD risk, a composite outcome of CHD and stroke, which is the focus of most primary prevention guidelines, was estimated in 40–79 years old adults.[26–28] However, PRSs may be more predictive of CVD risk in younger asymptomatic individuals than individuals with symptomatic CVD risk profiles.[5] While plaque build-up in the vasculature can begin during early adulthood, initially asymptomatically, progressing thereafter,[61, 62] identifying the earliest indication of a predisposition to such build-up, including genetic predisposition, may allow early preventative action to be taken in high-risk individuals. Furthermore, in the present study, we found associations limited to HbA1c and SBP PRSs with CVD risk but not with other cardiometabolic trait PRSs. Modest SNP effects on DBP levels have been previously reported.[18, 63]
In the present study, we derived PRSs using summary data that may be tailored towards African-ancestry individuals, i.e., target populations where there may be negligible differences in LD or causal allele frequencies with African-origin UK populations.[9] However, as underlying genetic ancestry may influence the established genetic associations with complex diseases, local genetic ancestry (characterized as ancestral states at each genetic locus) estimates should be included in regression models. In addition, larger base, target and validation GWASs in African-ancestry data are required to increase predictive power of PRSs in African-ancestry individuals. Lastly, in individuals with low polygenic risk, rare variant screening may enhance risk prediction for common diseases and guide appropriate risk stratification.[64]